Dichroic polarizer

ABSTRACT

The invention belongs to polarizing devices and can be used in lighting equipment, manufacturing construction-material glasses, and in displays. 
     The proposed dichroic polarizer includes a substrate and at least one layer dichroically absorbing electromagnetic radiation, into which two reflecting coatings are introduced, at least one of which is made partially transmitting. The layer dichroically absorbing electromagnetic radiation is located between the reflecting coatings. The materials and thicknesses of layers of both the dichroically absorbing electromagnetic radiation and the reflecting coatings are selected from the requirements to obtain, at the exit of the dichroic polarizer, an interference minimum for the absorbing component of electromagnetic radiation for, at least, one wavelength value. 
     The invention leads to increasing efficiency of dichroic polarizers at the expense of increasing degree of polarization of electromagnetic radiation leaving the polarizer, while high transmission (reflection) coefficient for the non-absorbed component is preserved. 5 formulas, 3 illustrations.

The invention belongs to polarizing devices and can be used in lightingequipment; manufacturing construction-material glasses and opticalinstruments, for example, spectrophotometers and displays.

The action of dichroic polarizers considered within the framework of theproposed invention is based on the property of a number of materialsusually termed dichroic, to differently absorb orthogonallinearly-polarized components of electromagnetic radiation. Dichroicfilm polarizers termed polaroids or polarizing light filters are themost widely applied. To create them, materials containing molecules orparticles (for example, microcrystals) are commonly used which, alongwith strong absorption, have strong dichroism in a wide range ofwavelengths. As a rule, these molecules or particles have extendedshapes, so orientation of molecules or particles is performed duringmanufacturing a polarizer in the certain (chosen) direction, also knownas the absorption axis. The transmission plane of the polarizer (thepolarizer plane) is then located perpendicularly to the absorption axis.The absorption degree of the components depends on orientation of theelectrical vector oscillation relative to the chosen direction. Whenconsidering the functioning of polarizers, it is convenient to designatethe orthogonally polarized components according to the degree of theirabsorption. Further, the terms absorbed (parasitic) component andnon-absorbed (the useful component) will be used.

To estimate the efficiency (quality) of polarizers, including dichroicpolarizers, and to compare between them, their polarizing abilities(degree of polarization) are normally used, which are determined usingvarious methods (A. I. Vanyurikhin, V. P. Gerchanovskaya, “Opticalpolarizing devices”, Kiev, Tekhnika, 1984 [1], page 23, in Russian).Further, the degree of polarization will mean the value determined, fora transmissive polarizer, via the energy transmission coefficients T₁and T₂ for the non-absorbed and the absorbed orthogonally polarizedcomponent respectively:P=(T ₁−T₂)/(T ₁ +T ₂),while, for the reflective polarizer, through the energy-relatedreflection coefficients R₁ and R₂ for the non-absorbed and the absorbedorthogonally polarized component respectively:P=(R ₁ −R ₂)/(R ₁ +R ₂)

Dichroic polarizers are known consisting of polymeric films stronglystretched in one direction and containing dichroic molecules, whichbecome oriented during stretching, for example, the iodine-polyvinylpolarizers based on polyvinyl alcohol ([1], pages 37-42). Thesepolarizers are multilayer films including, along with the polarizinglayer, also the reinforcing, gluing, and protecting layers. The basicdisadvantage of the specified film polarizers is rather high labor inputrequired for their manufacturing.

The polarizer closest in the technical basis to the one described hereinis the dichroic polarizer including a substrate on which a molecularlyoriented layer is deposited which has been obtained from organic dyewhich is in the lyotropic liquid crystal state (application PCT94/05493, Cl. C09B31/147, 1994). Use of such dyes allows to considerablysimplify the technology of manufacturing; dichroic polarizers, and tolower their cost accordingly, but the dichroic polarizers thus obtaineddo not have the sufficient degree of polarization.

The purpose of the invention is to increase the efficiency of a dichroicpolarizer at the expense of increasing the polarization degree ofelectromagnetic radiation, while preserving the high transmission(reflection) coefficient for the non-absorbed component.

The purpose set herein is achieved because, in a dichroic polarizercontaining a substrate and a layer of a dichroic material, tworeflecting coatings are introduced, at least one of which is madepartially transmitting, and the dichroically absorbing layer is locatedbetween the two reflecting coatings. Such a multilayer structure allowsto obtain multipath interference and resembles the Fabry-Perotinterferometer.

The dichroic polarizer can be implementing as a reflective one, and oneof the reflecting coatings will in this case be made completelyreflecting, while the second will be partially transmitting. Then, thefirst coating to be deposited from the substrate side may be either thereflecting one (completely reflecting), or the partially transmittingone.

The multipath interference results in obtaining, at the exit of thedichroic polarizer, interference maxima, minima, as well as intermediateintensity values, depending on thicknesses and materials of layers andcoatings constituting the polarizer.

Analysis of the influence of interference picture at the exit of theproposed polarizer on the polarization degree of radiation has shownthat, when an interference maximum of intensity is obtained, there is anincrease in either the energy-related transmission coefficient or, inthe other polarizer type (reflective rather than transmissive), thereflection coefficient, for both the absorbed and the non-absorbedcomponents. Thus the ratio of intensities of the transmitted (orreflected) radiation of the orthogonally polarized components decreases,and the degree of polarization decreases accordingly. Although thisincreases transmission (reflection) of the polarizer, this is not soimportant as reduction of the polarization degree.

When an interference minimum is obtained at the exit of a polarizer, theintensity is reduced of both orthogonally polarized components. However,both the calculations and the experimental data have shown that it ispossible to reduce the intensity of the absorbed components moresignificantly than that of the non-absorbed components. Although thiscauses some reduction in transmission (reflection) of the polarizer, thedegree of polarization substantially increases.

It is therefore relevant that the materials and layer thicknesses of thedichroic polarizer should be chosen from the requirement to obtain, atthe polarizer exit, an interference minimum for the absorbed componentsfor at least one wavelength of the electromagnetic radiation.

The wavelength for which an interference minimum should be obtained canbe set at, for example, the wavelength corresponding to the middle ofthe used spectral range.

The width of the used spectral range is then determined from thefollowing considerations.

The condition of obtaining an interference minimum at the exit of adichroic polarizer can be written as:Δ=mλ+λ/2,where Δ is the difference in the path lengths of two beams reflectedfrom the reflecting coatings when the beams leave the polarizer, m isthe order of interference, λ is the light wavelength. With a sufficientdegree of accuracy, the interference minimum also appears for theneighboring wavelengths, for which the path length difference a differsby no more than 10%. For larger orders of interference (m=10-50), i.e.when the thickness of the layer dichroically absorbing electromagneticradiation is large enough, the condition of 10% difference in the pathlength is satisfied for a very narrow range of wavelengths, so thepolarizer can be used only as a narrowband one. When the order ofinterference is zero (m=0), i.e. for small enough thickness of layerdichroically absorbing electromagnetic radiation, this condition issatisfied for a wider wavelength range. For example, if 550 is taken tobe the basic wavelength for which the equality (3) is valid, therequirement to obtain an interference minimum will be satisfied for,practically, the entire visible range. Hence, when thickness of thedichroically absorbing layer is comparable to the radiation wavelength,a broadband polarizer can be obtained.

From the theory of interference, it is known that, to obtain aninterference minimum, the optical path length difference betweeninterfering beams should be (λ/2+mλ), which is an odd number ofhalf-waves.

To ensure such path length difference, the thickness of the dichroicallyabsorbing layer is determined for at least one wavelength from theequality λ/4+λ/2=λ/ 4(1+2m).

The outcome of interference is largely influenced by the ratio ofamplitude values of the interfering beams. It is known that the minimalintensity value can be obtained when the amplitudes are equal.Therefore, it is relevant to make the amplitude values of theinterfering beams for the absorbed components as close as possible toeach other, which would provide maximal mutual cancellation of beams ofthese components. Simultaneously, one should ensure a significantdifference between the amplitudes of the interfering beams for thenon-absorbed components, which will practically exclude the opportunityfor these beams to interfere, i.e. intensities of the non-absorbedcomponents will not be appreciably reduced. If both requirements aresatisfied, increase in the polarization degree will be ensured, which ismore important than some decrease in transmission (reflection) of thepolarizer.

From the above considerations, it is relevant that the thickness h ofthe dichroically absorbing layer was chosen from the requirement for thefollowing equality to be valid for at least for one wavelength λ:hn=mλ+λ/4=(2m+1)*λ/4,where n is the is refraction coefficient of the dichroically absorbinglayer, and m is an integer,while the thickness and the material of reflecting coatings are chosenfrom the requirement to ensure, for the absorbed components, equality orapproximate (to within 10-20%) equality of amplitudes for at least twointerfering beams for at least one wavelength.

The reflecting coatings can be made either of metal, or manufacturedfrom multilayer dielectric mirrors consisting of alternating layers ofmaterials with high and low refraction coefficients.

The metal coatings are easy enough to deposit, for example, by thermalevaporation in vacuum. But then, light is absorbed in such coatings,which reduces transmission (reflection) of the polarizer. For thesecoatings, aluminium (Al), silver (Ag), and other metals can be used.

In case of multilayer dielectric mirrors, light is not absorb in them,but the process of their deposition is rather complex andlabor-consuming. For these coatings, TiO₂, MgO, ZnS, ZnSe, or ZrO₂, orpolymers can be used as the high refraction coefficient materials. Asthe low refraction coefficient materials, SiO₂, Al₂O₃, CaF₂, BaF₂, AlN,BN, or polymers can be used.

The following standard methods can be used to deposit reflectingcoatings: thermal evaporation in vaccum, deposition in vapor withsubsequent thermal processing, magnetronic dispersion, and others.

As a material for manufacturing the dichroically absorbing layer, anydichroically absorbing material can in principle be used, which can beshaped as a layer with the thickness comparable to the wavelength, inparticular, equal to λ/4. However, it is more relevent to use amolecularly oriented organic dye which is in the iyotropic liquidcrystalline state, from the following series:

The specified organic dyes allow to orient the dichroic dye moleculesdirectly during layer deposition. Thus, the technological process ofobtaining dichroic polarizers becomes considerably simpler, and,consequently, its cost decreases.

To deposit a layer dichroically absorbing electromagnetic radiation, thefollowing standard methods can be applied; deposition by a platen, by adoctor knife, by a doctor in the form of a non-rotating cylinder,deposition using a slit spinneret or die, etc.

The invention is illustrated by FIGS. 1-3. In FIG. 1, a scheme is shownof a dichroic polarizer according to the prototype. In FIG. 2, a schemeof a reflective-type dichroic polarizer is shown according to theinvention. In FIG. 3, a scheme of a transmitted-light dichroic polarizeraccording to the invention is shown.

In FIG. 1, the scheme of a dichroic polarizer according to the prototypeis presented including a layer 1 dichroically absorbing electromagneticradiation and deposited onto a substrate 2. In the dichroic polarizeraccording to the prototype, non-polarized electromagnetic radiation 3passes the layer 1 dichroically absorbing electromagnetic radiation anddeposited on the substrate 2, and becomes the linearly polarizedelectromagnetic radiation 4.

Analysis of properties of the prototype dichroic polarizer has shownthat, when the thickness of the layer 1 dichroically absorbingelectromagnetic radiation, is 50 nm, for the polarization degree of 80%,transmission of the useful polarized component by the dichroic polarizeris 90%. When the thickness of the layer 1 dichroically absorbingelectromagnetic radiation is 500 nm, for the polarization degree of 90%,transmission of the useful polarized component by the dichroic polarizeris 80%. When the thickness of the layer 1 dichroically absorbingelectromagnetic radiation is 2000 nm, for the degree polarization of99%, transmission of the useful polarized component by the dichroicpolarizer is 50%.

In FIG. 2, a scheme of a dichroic reflective-type polarizer according tothe invention is presented including a layer 1 dichroically absorbingelectromagnetic radiation, a layer 5 completely reflectingelectromagnetic radiation, and a layer 6 partially reflectingelectromagnetic radiation. All layers are consecutively deposited onto asubstrate 2.

Operation of the proposed dichroic reflective polarizer can be explainedas follows. The non-polarized electromagnetic radiation consists of twolinearly polarized components 7 and 8, with their polarization planesmutually perpendicular (these two components are conventionally shownapart from each other in FIGS. 2 and 3 for better presentation andunderstanding). The absorbed and not further used component 7, which ispolarized parallel to the absorption axis of the layer 1 dichroicallyabsorbing electromagnetic radiation, is partially reflected from thelayer 6 partially reflecting electromagnetic radiation, and forms thebeam 9. The other part of energy of the component 7 passes through thelayer 1 dichroically absorbing electromagnetic radiation, and, afterbeing reflected from the layer 5 completely reflecting electromagneticradiation, passes the layer 1 once again and then the layer 6 formingthe beam 10. The reflected beams 9 and 10 are polarized identically tothe initial component 7. The thickness of the layer 1 is chosen so asthe optical path length difference between beams 9 and 10 becomes an oddnumber of half-waves of polarized electromagnetic radiation, where thewavelength corresponds to the middle of the used spectral range. In thiscase, interference of the beams 9 and 10 result in their mutualweakening, and the complete cancellation in the optimum case. Completemutual cancellation of the beams 9 and 10 is achieved if the intensities(amplitudes) of the beams 9 and 10 have either identical or closevalues, which can be achieved by optimally selecting reflectioncoefficients of the reflecting layers 5 and 6. The reflecting layer 5and 6 can be made of metal, semiconductor or dielectric, and be eithersingle-layer or multilayer.

The other further used linearly polarized component 8 non-absorbed inthe layer 1, which is polarized perpendicularly to the optical axis(absorption axis) of the layer 1, is partially reflected form the layer6 partially reflecting electromagnetic radiation, and forms the beam 11.The other part of energy of the component 8 passes through the layer 1dichroically absorbing electromagnetic radiation, and, after beingreflected from the layer 5, passes the layer 1 once again and then thelayer 6, and forms the beam 12. The reflected beams 11 and 12 arepolarized identically to the initial component 8. Interference resultsin weakening the beams 11 and 12 considerably less than the beams 9 and10. This is caused by the fact that their intensities considerablydiffer because of the negligibly small absorption of the beam 12 in thelayer.

In FIG. 3, the scheme of a dichroic polarizer of a transmissive typeaccording to the invention is presented. The polarizer includes a layer1 dichroically absorbing electromagnetic radiation and layers 6 and 13partially reflecting electromagnetic radiation. All layers are depositedonto a substrate 2.

Operation of a dichroic transmissive-type polarizer of electromagneticradiation according to the invention can be explained as follows. Thenon-polarized electromagnetic radiation consists of two linearlypolarized components 7 and 8, with their polarization planes mutuallyperpendicular. Both of these components pass through the layer 6partially reflecting electromagnetic radiation, and then through thelayer 1 dichroically absorbing electromagnetic radiation. A part of theenergy of the components 7 and 8 passes through a layer 13 partiallyreflecting electromagnetic radiation, and forms beams 14 and 15respectively. The other part of energy of the components 7 and 8 isreflected from the layer 13 partially reflecting electromagneticradiation passes the layer 1, becomes reflected from the layer 6, onceagain passes the layers 1 and 13, and forms the beams 16 and 17respectively. The beams 15 and 17 are polarized identically to theinitial component 8, i.e., perpendicularly to the absorption axes. Thepassed beams 14 and 16 are polarized identically to the initialcomponent 7, i.e., parallel-perpendicular to the absorption axes.

The purpose of this invention is achieved because of unequal reductionof the differently polarized components 7 and 8 of electromagneticradiation passing through a dichroic polarizer during interference ofthe parts 9 and 10 of the component 7, as well as parts 11 and 12 of thecomponents 8. This is ensured by specially selecting thicknesses oflayer 1, 6 and 13. In particular, the optical thickness of the layer 1dichroically absorbing electromagnetic radiation should be an integernumber of wavelengths of polarized electromagnetic radiation. Bychanging thicknesses of the layers 13 and 5 partially reflectingelectromagnetic radiation, it is possible to select the values ofreflection coefficients of these layers optimum for increasing thedichroic polarizer efficiency. A criterion for choosing the reflectioncoefficients of the layers 13 and 5 can be, for example, the maximalcontrast of the dichroic polarizer. The optimum thicknesses of thelayers 13 and 6 do not affect the basis of the invention.

The layers 13 and 6 partially, reflecting electromagnetic radiation canbe made of metal or a multilayer dielectric, which does not affect thebasis of the invention.

Examples of specific embodiments of the dichroic polarizer are givenbelow.

EXAMPLE 1

A dichroic polarizer of the reflective type according to the invention(FIG. 2) for polarization in the visible (light) wavelength range, i.e.for the wavelengths band of 400-700 nm, is made as follows. On a glasssubstrate, the following layers are consecutively deposited: analuminium, strongly reflecting layer of 100 nm thickness (depositedusing thermal evaporation in vacuum); then a 50 nm thick layerdichroically absorbing electromagnetic radiation made of a mixture ofdyes . . . of Formulas 1,2,3; and then a 2 nm thick aluminium layerpartially reflecting electromagnetic radiation.

Measurements have shown the polarizing ability in the dichroic polarizerthus manufactured to be 92%, the reflection of the useful polarizedcomponent by the dichroic polarizer being 90%. A similar polarizingparameter in the prototype deposited onto a mirror was 80% for the samedyes and with the same thickness, and reflection of the useful polarizedcomponent by the dichroic polarizer was 90%.

EXAMPLE 2

A dichroic reflective-type electromagnetic radiation polarizer (FIG. 2)polarizing in the visible (light) wavelength range is manufactured asfollows. A strongly reflecting layer with 98% reflection coefficient inthe 490-510 nm wavelength range is deposited onto a glass plate as amultilayer dielectric coating. This coating is made of alternating MgF₂and cryolite layers. On top of this strongly reflecting layer, a 120 nmthick layer is deposited which dichroically absorbs electromagneticradiation and is made of oriented dye of Formula II. Then, a layer isdeposited partially reflecting electromagnetic radiation, withreflection coefficient of 28%, also made of MgF₂ and cryolite layers.

Measurements have shown the polarizing ability in the dichroic polarizerthus manufactured to be 95% in the 490-510 nm wavelength range, thereflection of the useful polarized component by the dichroic polarizerbeing 90%. The polarizing ability in the prototype deposited onto amirror was 85%, the reflection of the useful polarized component by thedichroic polarizer being 90%.

EXAMPLE 3

A dichroic transmitted-light electromagnetic radiation polarizer (FIG.3) polarizing in the wavelength region of 620-640 nm is manufactured asfollows. A 20 nm thick, partially reflecting aluminium layer isdeposited onto a glass plate (deposition using thermal evaporation invacuum). Then a 140 nm thick layer dichroically absorbingelectromagnetic radiation made of oriented dye of Formula IV isdeposited. Finally, the second 20 nm thick aluminium layer partiallyreflecting electromagnetic radiation is deposited.

Measurements have shown the polarizing ability in the dichroic polarizerthus manufactured to be 98%, the reflection of the useful polarizedcomponent by the dichroic polarizer being 80%. The polarizing ability inthe prototype was 86%, with 82% transmission of the useful polarizedcomponent by the dichroic polarizer.

EXAMPLE 4

A dichroic transmitted-light electromagnetic radiation polarizeraccording to the invention (FIG. 3) polarizing in the near infraredwavelength range is manufactured as follows. A layer partiallyreflecting in the 700-1200 nm wavelength range having the reflectioncoefficient of 40-55% is deposited onto a glass plate as a multilayerdielectric coating made of layers of zinc sulfite and ammoniumphosphate. On top of this strongly reflecting layer, a 180 nm thicklayer dichroically absorbing electromagnetic radiation made of orienteddye of Formula X is deposited, and then a layer partially reflectingelectromagnetic radiation with the reflection coefficient of 28%, alsomade of layers of zinc sulfite and ammonium phosphate.

Measurements have shown the polarizing ability in the manufactureddichroic polarizer to be 92% in the wavelength range of 700-1200 nm, thereflection of the useful polarized component by the dichroic polarizerbeing 80%.

The polarizing ability of the prototype was 75%, with 80% reflection ofthe useful polarized components by the dichroic polarizer.

Thus all the examples demonstrate the enhancement of the dichroicpolarizer efficiency due to the increasing of the polarization degree ofthe electromagnetic radiation admitted and with the same value of thetransmittance (reflectance) coefficient for the non-absorbed component.

1. A dichroic polarizer comprising: a substrate, two reflectivecoatings, and a layer dichroically absorbing electromagnetic radiation,wherein at least one of the reflective coatings is partiallytransmitting, and the layer dichroically absorbing electromagneticradiation is located between the two reflective coatings.
 2. Thedichroic polarizer of claim 1, wherein both reflective coatings are madepartially transmitting.
 3. The dichroic polarizer of claim 1, whereinmaterial and thickness of the layer dichroically absorbingelectromagnetic radiation are chosen from the requirement to obtain, atthe exit of the dichroic polarizer, an interference minimum for theabsorbing component of electromagnetic radiation for at least onewavelength range.
 4. The dichroic polarizer of claim 1, wherein at leastone of the reflective coatings is made of metal.
 5. The dichroicpolarizer of claim 1, wherein at least one of the reflective coatings ismade of multilayer dielectric mirror of the interchanged layers ofmaterials with high and low refraction coefficients.
 6. The dichroicpolarizer of claim 1, wherein the layer dichroically absorbingelectromagnetic radiation is made of an oriented layer of at least onedichroic dye applied from the lyotropic liquid crystalline state.
 7. Thedichroic polarizer of claim 2, wherein at least one of the reflectivecoatings is made of metal.
 8. The dichroic polarizer of claim 2, whereinat least one of the reflective coatings is made of multilayer dielectricmirror of the interchanged layers of materials with high and lowrefraction coefficients.
 9. The dichroic polarizer of any one of claim 6wherein said dichroic dye is selected from the group consisting ofmolecules having the following formulas I-X:

wherein n is an integer in the range of 2 to 4, and M is a cation;

wherein n is an integer equal to 2, and M is a cation;

wherein n is an integer equal to 2 or 3, X is S or O, and M is a cation;

wherein R is H or CF₃, X is individually selected from the group of H,Br, and SO₃M; n is an integer in the range of 1 to 3, M is a cation, R′is individually selected from the group consisting of H,

wherein R″=H, Cl, and

wherein n is an integer in the range of 2 to 4, M is a cation;

wherein n is an integer equal to 2, and M is a cation;

wherein n is an integer equal to 2, and M is a cation;

wherein n is an integer equal to 2 or 3, and M is a cation;

where R is individually selected from the group consisting of H, Cl,Alk, and OAlk, n is an integer equal to 2, and M is a cation; and

wherein R is individually selected from the group consisting of H, OAlk,NHR′, Cl, and Br; X is individually selected from the group consistingof O, NH, and CH₂; n is an integer equal to 2, and M is a cation.